1. Field of the Invention
The present invention relates to a sliding machine capable of significantly reducing friction coefficient, sliding resistance, and the like that are applied between sliding surfaces using a combination of an amorphous carbon film containing chromium (Cr) which is a specific element (chromium-containing DLC film), and a lubricating oil containing an oil-soluble molybdenum compound having a specific chemical structure.
2. Description of Related Art
Many machines have sliding members which move relative to each other while coming into sliding contact with each other. In such machines having sliding members (in the specification, referred to as “sliding machines”), by reducing resistance (sliding resistance) applied to the sliding parts, performance is enhanced and energy necessary for operations is reduced. The reduction in the sliding resistance is typically achieved by a reduction in the friction coefficient of friction applied between sliding surfaces.
The friction coefficient of friction applied between the sliding surfaces varies depending on the surface states of the sliding surfaces and the lubricating state between the sliding surfaces. Therefore, in order to achieve the reduction in the friction coefficient, the surface modification of the sliding surfaces and the improvement of a lubricant (lubricating oil) supplied between the sliding surfaces are considered. There are various methods for the surface modification of the sliding surfaces. However, in many cases, an amorphous carbon film (a so-called diamond-like carbon (DLC) film) which achieves a reduction in the degree of friction and has excellent wear resistance is formed on the sliding surfaces. In addition, the lubricant is also improved in various ways depending on the type of sliding machine, use environment, and the like, and typically, the improvement may correspond to mixing an additive which is effective in reducing friction.
However, the DLC film which is considered to be effective in reducing friction varies in property between a dry type and a wet type. Moreover, the sliding property of the DLC film in the wet type varies depending on the type of the applied lubricating oil. Here, an optimal combination of a specific DLC film and a specific lubricating oil is important to achieve a reduction in the friction coefficient. Suggestions related to this are, for example, the following patents.
Japanese Patent Application Publication No. 2001-316686 (JP 2001-316686 A) suggests a combination of a DLC film containing Mo or Ti and a lubricating oil containing 500 ppm of molybdenum dithiocarbamate (MoDTC). In addition, WO2005/14763 suggests a combination of a general DLC film which does not contain metal elements and the like and a lubricating oil which contains a sulfur-containing molybdenum complex (MoDTC) in a proportion of 9.9 mass % in terms of Mo content. The MoDTC used in JP 2001-316686 A and WO2005/14763 is an additive of a well-known engine oil and is made of binuclear molybdenum. Japanese Patent Application Publication No. 2011-252073 (JP 2011-252073 A) suggests a combination of a lubricant which contains an organic molybdenum compound instead of the MoDTC, in which the mass ratio (N/Mo) of nitrogen and molybdenum is in a predetermined range, and a H (20%)-containing DLC film.
Japanese Patent Application Publication No. 2004-339486 (JP 2004-339486 A) (EP Patent No. EP1462508B1) suggests a combination of a general DLC film which does not include metal elements and the like and a lubricating oil in which trinuclear molybdenum dithiocarbamate is added to base oil in a proportion of 550 ppm in terms of Mo content. However, in JP 2004-339486 A (EP Patent No. EP1462508B1), only the intent that the friction coefficient is reduced by the combination is described, and the mechanism is not clarified at all. In addition, the friction coefficient obtained by the combination is only about 0.1, and the reduction in the friction coefficient is still insufficient.
As described above, although suggestions for a reduction in the friction coefficient using an appropriate combination of a DLC film and a lubricating oil have been provided hitherto, the suggestions do not achieve a significant reduction in the friction coefficient. In addition, mechanisms of changing the friction coefficient using the combination of a DLC film and a lubricating oil, and the like are not clarified yet.